Torsion springs are well known devices utilized for, among other things, providing resistive force against both torsional and linear motion. Some of the advantages of torsion springs over other type springs are lower cost, smaller packaging and better reliability. Typical prior torsion springs have been made by concentrically positioning cylindrical sleeves of metal of different diameters and securing a resilient material, such as an elastomer, therebetween. The inner sleeve is secured to a first element which is to be resiliently carried for torsional movement and the outer cylindrical sleeve is secured to another or second element such as a housing and with respect to which the first element has relative torsional movement. This type of torsion spring is known as an elastomeric torsion spring.
FIG. 1 illustrates a prior elastomeric torsion spring 10 which includes an elastomeric member 12 disposed between and bonded to an inner sleeve 14 and an outer shell 16. Typically, outer shell 16 is anchored and inner sleeve 14 is keyed so that a shaft 18 extending therethrough can be utilized to rotate the inner sleeve 14 with respect to the outer shell 16. Under these circumstances, shaft 18 and outer shell 16 must be connected to parts of a device which are intended to move relative to one another.
FIG. 2 illustrates an elastomeric torsion spring having an outer shell 26 which rotates in a direction opposite to the direction of rotation of an inner sleeve 22. A center shaft 20 of the inner sleeve 22 is keyed, thereby allowing the shaft 20 to be rotated by a keyed rod (not shown). The outer shell 26 therefore must be connected or anchored to something which has torsional movement with respect to the inner shaft. Each spring arranged and utilized in such a manner has a particular windup or spring rate curve to describe the amount of torque provided by the spring for any given angular displacement or torsional movement between the inner sleeve and the outer shell.
FIG. 3 illustrates a torque versus windup curve of a typical prior torsion spring such as is shown in FIG. 1. Designers who incorporate torsion springs such as illustrated in FIG. 1 utilize the torque versus windup characteristics to anticipate the amount of torque produced by the spring at any particular given amount of angular displacement. It can be seen from FIG. 3 that the torque provided by the spring generally increases as the windup of the spring increases. It can also be seen that the torque is not directly (linearly) proportional to the windup, with the nonlinearity becoming more pronounced at higher angular deflections.
There are certain applications in which it is advantageous to utilize a torsion spring to provide torque which is constant over a particular distance or angular displacement. A device which accomplishes this is therefore desirable. In addition, since torsional springs heretofore provide a singular torque vs. windup curve, it has previously been necessary to change them out in order to achieve varying amounts of torque for a given displacement. It is not always practical however to change springs in a structure in order to accomplish this objective. Likewise, a constant force mechanism utilizing prior torsion springs requires changing of the spring in order to provide different force levels. A constant force mechanism which utilizes a torsion spring having a multiplicity of linear torque vs. windup curves can provide differing force levels without changing springs, and is therefore also highly desirable.